Has anyone successfully done this? I've tried nosplash, quiet, and what seems to be every other combination of boot arguments, and I still can't get it to boot. I really want to use this thing with Ubuntu Linux.

2) make SURE you burn the disc image properly. If you're just burning the image to the disc you're doing it wrong, it has to essentially be cloned.
Kbase article: http://support.apple.com/kb/ht2087

3) Make sure your boot cd-rom/dvd-rom/etc is working properly. Try burning the image at a slower speed if the drive you're reading from is older than the one you're burning with, and test/make sure the drive itself works. CD-rom drives, like any other piece of hardware, do die eventually, I've had several crap out on me over the years, and G4s are old...

4) if all else fails, try the alternate images.

Also, this may be another "duh" thing, but just holding down "c" after the boot chime will boot from CD instead of hdd. Good luck!

Alternate Install image is on the same page (check the links above). You may want to try redownloading ore reburning the disc image, you may have just had some corruption in the download (maybe pick a different mirror if that's an option too).

At what point is it failing? Are you able to complete the install itself?